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+\documentclass[12pt]{article}
+
+\begin{document}
+
+\begin{eqnarray*}
+  E^{ZBL}_{ij} & = & \frac{1}{4\pi\epsilon_0} \frac{Z_i Z_j \,e^2}{r_{ij}} \phi(r_{ij}/a)+ S(r_{ij})\\
+  a & = & \frac{0.46850}{Z_{i}^{0.23} + Z_{j}^{0.23}}\\
+  \phi(x) & = & 0.18175e^{-3.19980x} + 0.50986e^{-0.94229x} + 0.28022e^{-0.40290x} + 0.02817e^{-0.20162x}\\
+ \end{eqnarray*}                           
+
+\end{document}